The invention relates to an electroluminescent device comprising:
an electroluminescent element having an electroluminescent organic layer disposed between a
hole-injecting electrode and an electron-injecting electrode, and
a housing enclosing said electroluminescent element, said housing comprising
a first shaped part having a first sealing surface,
a box-like second shaped part having a second sealing surface with an -inner perimeter,
said electroluminescent element being mounted on said first shaped part,
said first and second shaped parts being connected to each other by means of a seal comprising
a closed ring of a sealing material extending between said first and second sealing surfaces.
The invention further relates to a method of manufacturing such an electroluminescent device.
An electroluminescent (EL) device is a device which, while making use of the phenomenon of electroluminescence, emits light when the device is suitably connected to a power supply. If the light emission originates from an organic material, said device is referred to as an organic electroluminescent device. An organic EL device may be used, inter alia, as a thin light source having a large luminous surface area, such as a backlight for a liquid crystal display or a watch. An organic EL device may also be used as a display if the EL device comprises a number of EL elements, which may or may not be independently addressable.
The use of organic layers as EL layers in an EL element is known. Known organic layers generally comprise a conjugated, luminescent compound. Said compound may be a low-molecular dye, such as a coumarin, or a high-molecular compound, such as a (poly)phenylenevinylene. The EL element also comprises two electrodes, which are in contact with the organic layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. If the EL element is in the form of a stack of layers, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Known cathode materials are, inter alia, Al, Yb, Mg:Ag, Li:Al or Ca. Known anode materials are, in addition to ITO, for example, gold and platinum. If necessary, the EL element may comprise additional organic layers, for example, of an oxadiazole or a tertiary amine, which serve to improve the charge transport or the charge injection.
An EL device of the type mentioned in the opening paragraph is disclosed in PCT application WO 98/53644. In said known organic device, the sealing material is low-melting metal or a low-melting metal alloy. This metal material is used according to PCT application WO 98/53644. Experiments showed that the housing should be airtight and waterproof to such an extent that organic sealing materials cannot be employed as barrier materials in the housing. Even epoxy-based adhesives and high-molecular, halogenated or non-halogenated hydrocarbons, which are reputed to be the best barrier materials within the class of organic materials, are unsuitable. Furthermore, apart from the worse barrier properties, the large difference between the coefficients of expansion of organic sealing materials and, for example, glass, and the resulting bonding problems proved to be disadvantageous.
However, although in some applications the known device proves to be satisfactory, there are applications, especially for very small pitch EL devices, or devices for which the inherent electrical conductivity of the metal poses a problem, for which the use of metal sealing material is not satisfactory.
The invention aims to provide a device as described in the opening paragraph, which overcomes the problems of using metal sealing material.
To this end, the device is characterized in that the sealing material comprises an organic sealing material, and in the direction towards the interior of the second part, the sealing material maximally extends to the inner perimeter of the second sealing surface.
The inventors have realized that, although PCT application WO 98/53644 states that xe2x80x98organic materials cannot be used as barrier materialsxe2x80x99, organic materials can in fact be used as sealing materials, provided that specific conditions, as described above, are met.
Hitherto, when organic sealing materials were used, such materials were provided on one of the parts in a relatively thick layer, whereafter the two parts were pressed upon each other. The seal had a thickness of several tens of micrometers, and inherently some sealing material was pushed inside the housing. This gives rise to at least two problems.
Almost all organic materials comprise gases. They also permit diffusion, particularly of moisture through the material. The outgassing of said organic materials and the diffusion through the organic materials gives rise to the rapid degradation of the EL element as described in PCT application WO 98/53644.
In the device in accordance with the invention, the sealing material between the first and the second part comprises an organic sealing material, and in the direction towards the interior of the second part, the sealing material maximally extends to the inner perimeter of the second sealing surface. In said device, substantially no sealing material is present inside the housing. This strongly reduces the amount of gas that is released by the organic material due to outgassing.
In the device in accordance with the invention, the organic sealing material preferably has a thickness of is less than 100 micrometers at any position between the first and second sealing surfaces. Below this limit, capillary action can be used to deposit the sealing material between the sealing surfaces.
In the device in accordance with the invention, the organic sealing material preferably has a thickness of less than 10 micrometer, at any position between the first and second sealing surfaces, and a distance of at least 0.2 mm between the outer and inner perimeter of the seal throughout the ring. The strongly reduced thickness of the sealing material in combination with a width of at least 0.2 mm of the ring provides a strongly increased resistance to diffusion of moisture through the seal.
Further advantages are a reduced amount of material used. The reduced thickness of the seal also alleviates bonding problems and increases the strength of the seal.
Preferably, the organic sealing material is chosen from the group of epoxy-based adhesives and high-molecular, halogenated or non-halogenated hydrocarbons. The diffusion of moisture through such materials is relatively small.
Preferably, the organic sealing material comprises inorganic particles. Such particles may be, for example, Al2O3, SiO2 or Mg-silicate particles. The presence of such particles effectively increases the diffusion path length for moisture, because the moisture does not diffuse or diffuses much more slowly through inorganic materials. This increase of the diffusion path length decreases diffusion of moisture through the seal. The amount of organic sealing material (at an equal thickness of the seal) is also reduced, which reduces the amount of gas that may be released by the organic sealing material.
The method of the invention is characterized in that the connecting step comprises the steps of bringing the sealing surfaces of the first and the second part near or against each other; providing an organic sealing material around the outer perimeter of the contact area between the first and the second part and allowing the organic sealing material to be deposited between the sealing surfaces by capillary action. The method in accordance with the invention has the advantage that the transport of the sealing material is stopped as soon as the sealing material reaches the inner perimeter of the second sealing surface. Sealing material is therefore substantially absent in the inner part of the housing.
xe2x80x98Nearxe2x80x99 or xe2x80x98againstxe2x80x99 is understood to be direct contact, including a position at a distance conducive to capillary action, and comprising embodiments in which spacers are positioned in between the first and the second part,. When incorporated in the seal, said spacers may also decrease diffusion by increasing the effective diffusion path length.
Preferably, the sealing material is provided at a temperature at which no transport through capillary action takes place, which temperature is subsequently raised to such a value that transport of organic sealing material through capillary action takes place. The advantage of providing the sealing material at a cold xe2x80x98immobilexe2x80x99 temperaturexe2x80x99 and subsequently raising the temperature to a xe2x80x98capillary activexe2x80x99 temperature is that, prior to actual sealing, the provision of the sealing material can be checked.
As compared with the provision of sealing material in a layer on one or both sealing surfaces, much less material needs to be used. Due to inaccuracies and surface irregularities, an applied layer must usually have a larger thickness than the actual sealing layer to insure that sealing material touches both surfaces even at the positions where the gaps are largest. This means that a substantially larger sealing material layer (2xc3x97 micrometers) must be applied for an average seal thickness of x micrometer. Part or even a major part of the surplus sealing material will be pushed inside the housing. The method in accordance with the invention does not suffer from these problems, because substantially the entire sealing surface of the parts are provided with sealing material, without sealing material being pushed or otherwise finding its way inside the housing.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.